EXAFS study on the cause of enrichment of heavy REEs on bacterial cell surfaces

EXAFSを用いたバクテリアの細胞表面における重希土類元素の濃集機構に関する研究

高橋 嘉夫*; 山本 美香*; 山本 祐平; 田中 万也  

Takahashi, Yoshio*; Yamamoto, Mika*; Yamamoto, Yuhei; Tanaka, Kazuya

希土類元素の相対存在度のパターン(REEパターン)はさまざまな天然の物質で観測されるユニークな地球化学的トレーサーである。バクテリアに吸着したREEのパターンは重希土類元素(HREE)を濃集し、天然試料におけるバクテリア関与の指標に成りえる。本研究では広域X線吸収微細構造(EXAFS)とバクテリアへのREEの分配パターンを用いて、バクテリアの細胞表面へのHREEの濃集機構の解明を試みた。EXAFSの結果より、HREEは低REE/バクテリア比環境では多座リン酸基に結合しているが、高REE/バクテリア比ではカルボキシル基に結合していることが示された。一方、軽希土類元素及び中希土類元素は低REE/バクテリア比において低配座数のリン酸基に結合し、高REE/バクテリア比ではカルボキシル基に結合していることが示された。REE/バクテリア比の変化に伴うバクテリアへのREEの分配パターンの変化は、EXAFSの結果と整合的であった。バクテリアの細胞表面へのHREEの濃集は多座リン酸基による安定な錯体の形成が原因であることがEXAFSによって示された。多座リン酸基はバクテリア以外の天然試料では見られない特徴であり、本研究の結果はバクテリアが関与した試料のREEパターンがバイオマーカーとして利用できる可能性を示した。

Among various natural samples, rare earth element (REE) pattern of bacteria exhibits anomalous enrichment at heavy REE (HREE) part, which can be a signature of bacteria-related materials. In this study, REE binding site on the cell surface of a Gram-positive bacterium (Bacillus subtilis) responsible for the HREE enrichment has been identified by extended X-ray absorption fine structure (EXAFS) coupled with the variation of REE distribution pattern. EXAFS showed that (1) HREE is bound to multiple phosphate site at lower REE-bacteria ratio (=[REE]/[bac]), but the fraction coordinated to carboxylate increased as the increase in the ratio and (2) the binding sites of light and middle REE change from phosphate with lesser coordination number to carboxylate site as the [REE]/[bac] ratio increases. On the other hand, the enrichment of HREE in the REE distribution patterns of bacteria was less marked as the increase in the [REE]/[bac] ratio. This result is consistent with the EXAFS results, since REE pattern of multiple phosphate site exhibits monotonous increase for HREE, while phosphate with lesser coordination number and carboxylate site have maxima around Sm. Based on these results, it was clear that phosphate site is more stable than carboxylate site as the binding site for REE. The average bond lengths between REE and oxygen were compared among various REE sorbed on bacteria, showing that the bond length was much shorter for HREE than those extrapolated from the trend between La and Dy due to the selective binding of HREE to the multiple phosphate site. Based on the results, it is thought that materials having such phosphate site can induce anomalous HREE enrichment in natural systems. Compared with other possible host phases of REE such as metal oxides and humic substances, the multiple phosphate site is unique to bacteria and bacteria-related materials such as biofilm and microbial mats, which leads to the potential of REE pattern as a biomarker in natural samples.